The proposed research will use one of the most social vertebrate species known, the bluegill, as a model system for investigating how various hormones are involved in the expression of alternative reproductive behaviors. Bluegill display a complex mating behavior in which males take one of two mutually exclusive life history pathways, either delaying maturity to become "parental" males or maturing precociously as "cuckolders". The goal of the proposed research is to identify the physiological and environmental factors that influence the alternative reproductive behaviors inherent in these two life histories. To accomplish this goal, a series of three experiments are proposed. Experiment 1 will determine the environmental and/or social stimuli that trigger the dramatic endocrinological changes that occur among parental male bluegill immediately preceding, during, and immediately after spawning. In addition, Experiment 2 will determine the role(s) that different levels of steroids play in regulating the various components of parental male reproductive behavior (e.g., nest construction, spawning, parental care, and aggression against competitors or predators). Finally, Experiment 3 will determine how the social structure of a population, the production of androgens, and the choice between alternative reproductive behaviors of male bluegill are interrelated. This proposal represents a unique blend of endocrinology, behavioral ecology, and genetics in a multidisciplinary investigation of reproductive behavior. The proposed experiments combine field behavioral studies on wild and experimental bluegill populations with laboratory hormone determinations using radioimmunoassay procedures. The results of this study will help elucidate how reproductive behavior is influenced through hormonal control mechanisms and ultimately, how alternative reproductive behaviors evolve.